This invention relates to circuit breakers of the molded case, narrow width type such as is shown and described in U.S. Pat. No. 3081386 to M. F. Koenig et al dated Mar. 12, 1963 and owned by mesne assignments by the assignee herein. Circuit breakers of this type are normally utilized in residential and commercial applications up to 240 volts.
In circuit breakers of the aforementioned type, a movable contact is driven between open and closed positions with a stationary contact by an overcenter drive spring connected between the movable contact and a latch lever. An operating lever is pivotally moved to carry a pivot end of the movable contact back and forth across the center, or line of action, of the drive spring to effect contact separation in a manual mode. The latch lever is released in response to overload currents to carry the drive spring across the plane of the pivoted movable contact to effect contact separation in an automatic mode. The force provided by the drive spring also establishes contact pressure for the breaker contacts. In prior art circuit breakers of this type, the latch lever is pivoted such that its motion will carry the spring quickly overcenter of the plane of the movable contact while minimizing any foreshortening of the working length of this spring, thereby to quickly separate the breaker contacts while the contact pressure remains substantially constant prior to contact separation. Such operation follows recognized good electrical circuit switching techniques for primary switching devices.
It has been found, however, that under high currents such as high short circuit conditions, forces associated with the high currents exert an opening force on the movable contact which counteracts the contact pressure and tends to cause the contacts to separate in advance of the point at which the released latch lever carries the drive spring across the plane of the movable contact. The magnitude of this opening force is proportional to the current and determines the rate at which the contact force is reduced and ultimately overcome to effect contact separation. Early and rapid contact separation is desirable in high current interruption and inasmuch as circuit breakers are not primary switching devices subjected to high lifetime operations, it is not essential that contact life requirements of a circuit breaker meet those of primary switching devices. As mentioned above, primary switching devices maintain the contact pressure substantially constant during initial switch mechanism movement and abruptly reduce this pressure at the point at which the mechanism effects contact separation. For circuit breakers, however, reduction of the contact pressure early in the movement of the operating mechanism enables the forces associated with high currents to quickly overcome the contact pressure and thereby advantageously effect early and rapid separation of the contacts.
Circuit breakers of the aforementioned type utilize bimetallic elements to sense low magnitude overload currents whereby such currents generate heat in the bimetal which cause it to deflect uniformly along its length to operate a latch lever release system. Critical attention to the selection of the particular composition of the bimetal element is necessary to achieve the required travel for releasing the latch system within the confinements of the available space within the breaker. However, highly sensitive bimetal elements are susceptible to damage when subjected to high currents whereby the bimetal may take a "set" and not return to its original position upon removal of the current.